The use of screen devices for filtering extruded plastic material is well known. Screening is particularly useful when extruding recycled plastics in order to remove particles of dirt or foreign matter from the recycled material. Screen devices, however, are particularly subject to clogging when utilized in plastics recycling operations. Accordingly, the changing of screen devices at frequent intervals is necessary.
Mechanisms for changing screen devices associated with plastic extruders are well known. Typically, such a mechanism may include a slide assembly which moves a clean screen device into the path of the extrudate as it moves the used, clogged screen device to a standby station where it is replaced. The replacement should be done relatively rapidly so that a clean screen device is always available for insertion into the extruder. In fact, automatic replacement would be quite desirable, not only to minimize the replacement time, but also to minimize any inconvenience associated with manual replacement. However, with one exception, all of the known screen changers effect replacement of the dirty screen devices manually. The one exception is the automatic screen changer of my prior U.S. Pat. No. 4,082,487 which automatically ejects a used, clogged screen device and replaces it with a clean screen device.
While this automatic screen changer overcomes the problems associated with manual replacement of clogged screen devices, it, like the manual machines, requires the use of discrete screen devices, each of which typically includes a housing containing one or more screen elements and each of which must be uniquely configured in accordance with its intended use. More specifically, the dimensions of each screen device are determined by the apparatus with which it is to be used, while the number of screen elements included in each device and the mesh of each element are determined in accordance with the type of extrudate and the desired degree of filtering thereof.
Generally, these requirements present little, if any problem. Where, however, an installation employs different sizes of extruders and/or screen changers, or will be used to filter extrudates having varying filtering requirements, many different sizes and types of screen devices may have to be stored, with the attendant high cost and inconveniences usually associated with the stockpiling of different parts.